The present invention relates to a frequency stabilization circuit and, more particularly, to a frequency stabilization circuit for stabilizing an intermediate frequency signal from a received signal in a radio receiver such as a portable telephone.
FIG. 4 shows an example of a conventional frequency stabilization circuit of this type.
As shown in FIG. 4, the conventional frequency stabilization circuit comprises a voltage-controlled oscillator 13 serving as a local oscillator for generating a local oscillation signal S13, a mixer 1 which uses the local oscillation signal S13 from the voltage-controlled oscillator 13 to convert the frequency of an input radio frequency signal S1 and output an intermediate frequency signal S2, a limiter 2 for amplifying the intermediate frequency signal S2 output from the mixer 1 while limiting its amplitude, a reference oscillator 7 for oscillating a reference signal S7 having a frequency obtained by multiplying the frequency of the intermediate frequency signal S2 with an integer (N), a frequency divider 9 for dividing the frequency of the reference signal S7 into N, a frequency comparator 10 for comparing the frequency of an amplitude-limited, amplified intermediate frequency signal S3 with the frequency of a reference signal S8 obtained by dividing the frequency of the reference signal S7 into N, a controller 11 which outputs voltage data S11 for feedback-controlling the voltage-controlled oscillator 13 on the basis of frequency difference information S10 as the comparison result of the frequency comparator 10, and a D/A converter 12 for converting the voltage data S11 into a DC voltage.
With this arrangement, a frequency difference between the input signal S1 and the reference signal S7 is measured, and the oscillation frequency of the voltage-controlled oscillator 13 is feedback-controlled to stabilize the frequency of the amplified intermediate frequency signal S3 input to a demodulator 3.
In the use of the conventional frequency stabilization circuit for a portable telephone, when the reception electric field strength temporarily weakens upon movement of the portable telephone or the like, the SNR (Signal-to-Noise Ratio) of the output S3 from the limiter 2 decreases, and the frequency cannot be correctly compared and converges on an erroneous frequency, resulting in a reception error.